Automobiles include a drive force transmitting mechanism, which incorporates therein a constant-velocity joint for transmitting the rotational drive force of a drive shaft from a driven shaft to axles. The drive force transmitting mechanism includes, for example, a tripod constant-velocity joint interposed between the drive shaft and the driven shaft, which is capable of transmitting rotation at a constant velocity even when an angle is formed between the drive shaft and the driven shaft.
The tripod constant-velocity joint includes an outer member comprising a shank and a tubular member. The drive shaft is coupled to the shank, and an inner member is fitted over the distal end of the driven shaft and inserted into the tubular member. The tubular member has a plurality of guide grooves defined in an inner wall surface thereof at equally spaced intervals. Each of the guide grooves is defined by a bottom surface and two side walls rising substantially perpendicularly from the bottom surface.
The inner member has an annular portion and a plurality of trunnions projecting from an outer circumferential wall of the annular portion. The trunnions are disposed in facing relation to the respective guide grooves. Substantially annular rollers are rotatably fitted over the respective trunnions, with needle bearings (rolling bodies) interposed therebetween. When the rollers are inserted respectively into the guide grooves, the drive shaft and the driven shaft become coupled to each other. Since the circumferential side walls of the rollers are held in sliding contact with the side walls of the guide grooves, the driven shaft displaces the outer member.
Ordinary needle bearings have a main portion in the form of a constant-diameter cylindrical body, and a distal end portion leading to the main portion. The distal end portion has an arcuately curved cross-sectional shape (see, for example, Patent Document 1, FIG. 6). If a flange is provided on the inner circumferential wall of each of the rollers to accommodate a needle bearing, then a machining cutter for the rollers may become quickly deteriorated, and it is not easy to accommodate the needle bearing within the roller. According to Patent Document 1, it has been proposed to make both bottom surfaces of the needle bearing flat.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-176763